1. FIELD OF THE INVENTION
This invention relates to an epoxy resin composition excellent in toughness, thermal resistance and low residual stress.
2. DESCRIPTION OF THE PRIOR ART
Epoxy resins in general have excellent characteristics in curing property, adhesive property, mechanical strength, and chemical resistance, and are used in a wide field of applications including molding, lamination, adhesives, and matrix resins for fiber reinforced composite materials. On the other hand, however, epoxy resins have a defect of being brittle. Thus, they have difficulties in toughness in that when they are cured with conventional amines, acid anhydrides, or phenol compounds, the resulting cured product shows poor impact strength and low elongation at break. This causes a serious problem particularly when they are used in structural materials of aeroplanes, motorcars and so forth.
On the other hand, in discrete semiconductors such as condensers, diodes, transistors, thyristors and Hall elements and integrated circuits including IC and LSI, as the sealing material for protecting the semiconductors mechanically and electrically from external environment, epoxy resins are mainly used at present. However, epoxy resins have a problem of internal stress being developed by the difference of the linear expansion coefficient of the resins from that of silicon chips or lead frames, or by strain produced in curing. The internal stress gives rise to flaws and cracks in the protective film for semiconductor elements, and further causes development of cracks in the semiconductors themselves. Further, the internal stress causes the distortion of lead frames, leading to poor insulation between frames.
The insufficient toughness of previous epoxy resins is caused by their excessively high density of crosslinking formed when the resins are cured with conventional amines, acid anhydrides and phenol compounds. One conceivable method of reducing the crosslinking density to some extent is to use either of the epoxy resin and the curing agent to be compounded in excess of the other. In this case, however, since the molecular chain is terminated with the epoxy resin or the curing agent positioned at the molecular terminal in the curing reaction, the resulting product is greatly deteriorated in such properties as mechanical strength, solvent resistance and water resistance, and hence cannot be used in practice.
The development of internal stress mentioned above is mainly affected by the linear expansion coefficient and the flexural modulus of the resin. Accordingly, it has been previously proposed to add an inorganic filler to the resin in order to decrease the linear expansion coefficient, and to add to a certain kind of flexibility-imparting agent to decrease the flexural modulus.
However, addition of too much an amount of inorganic fillers causes serious problems including increased flexural modulus, decreased moisture resistance, and deteriorated flowing properties.
Addition of flexibility-imparting agents results in insufficient crosslinking density, and hence causes the decrease of thermal resistance and moisture resistance, and further leads to the increase of the linear thermal expansion coefficient.